Lentiviruses, a group of retroviruses that cause slow virus diseases in domestic animals and may be distantly related to newly described human retroviruses, are being intensively analyzed by immunological techniques and by molecular cloning technology. The proviral DNA of caprine arthritis-encephalitis virus (CAEV) has been cloned, and cloning of the DNA of equine infectious anemia virus (EIAV) is currently underway. Radioimmunoassays to the major structural proteins of these viruses are being used to study genetic relatedness of these agents to other retroviruses and as a sensitive means of analysis of virus transmission and sero-conversion under natural and experimental conditions. Hybridoma technology is being used to study the structural proteins of a transforming primate herpesvirus (Herpesvirus saimiri) and also to produce monoclonal antibodies to the proteins produced by normal and transforming onc genes.